Magnesium has a specific gravity (a density in g/cm3 at 20° C.) of 1.74 and is the lightest metal among the metals used as a structuring material. Consequently, in recent years, cases have been increasing where it is used as a material for portable apparatuses and motorcar components, both of which are required to be light-weight. As the currently employed method of producing a magnesium-alloy product, the injection casting process is mainly used, such as the die casting process, the thixomolding process, and another injection molding process.
In addition, a magnesium-alloy material having higher strength can be obtained by performing a plastic processing on a billet-shaped cast material obtained through the semicontinuous casting process such as the direct-chill (DC) casting process. However, a cast material obtained by the semicontinuous casting process has a large crystal-grain diameter. Therefore, it is difficult to perform the plastic processing, such as forging, drawing, and rolling, without a pretreatment. Consequently, it is known that it is necessary to heat the cast material again to carry out the extrusion operation under the hot condition in order to obtain fine crystal grains before performing the above-described plastic processing. The performing of such a hot extrusion increases the number of processes. In addition, the productivity decreases greatly because a magnesium alloy is an active metal and therefore it is necessary to determine the extrusion speed so that sufficient cooling can be performed at the time of the extrusion. In view of the above circumstances, Patent literature 1 has disclosed that the employment of the continuous casting using a movable casting mold enables the performing of the hot rolling without carrying out the extrusion operation in advance. On the other hand, Patent literature 2 has disclosed that a rolled wire can be obtained by rolling an ingot of magnesium alloy using grooved rolls under a specific rolling temperature condition.
Patent literature 1: Internationally published pamphlet 02/083341
Patent literature 2: the published Japanese patent application Tokukai 2004-124152.